Various configurations of electric cartridge heaters are known in the prior art. A typical cartridge heater includes a metal sheath around a resistance-wire heating element coiled around a core of insulating material. An insulating filler material with appropriate thermal conductivity and electrical insulating properties is used to fill the space between the coil and the sheath. Granulated magnesium oxide is typically used as the insulating filler material. After the sheath is filled, the sheath is subjected to compression forces, for example, by swaging. Compression compacts the granulated magnesium oxide and improves its dielectric and thermal conductivity properties. Lead wires may be attached to the coil before or after filling the sheath and may be held in place with an end plug made of materials such as Teflon, mica and silicone rubber. The lead wires become secured within the plug after swaging. The lead wires may then be potted with sealants to provide moisture resistance. Depending upon the intended application, cartridge heaters of varying sizes and voltage ratings may be required. U.S. Pat. No. 6,172,345, for example, discloses a high voltage cartridge heater which includes a core sleeve of pre-compacted insulating material.
With current manufacturing technology, it has proven to be a challenge to reliably produce high-voltage cartridge heaters for high moisture environments. Heaters in operation in high moisture environments are prone to dielectric breakdown and current leakage problems caused by the egress of moisture and water into the dielectric insulating material. In high moisture environments, dielectric integrity and current leakage must be kept within predetermined limits in order for the cartridge to meet certain industry standards, such as those standards established by Underwriters Laboratories, for example, the UL 471 standards.
One apparent reason for such problems is that the potting sealants and sealant methods used to seal the lead wire end of the cartridge do not provide adequate bonding with the lead wires and the sheath. Sealant materials, such as epoxy and silicone, for example, do not bond adequately with the stainless steel used for the construction of the sheath or with the silicone-coated lead wires. As a result, high-voltage cartridge heaters are traditionally only offered with sealants that do not qualify for certification for high moisture environments under the applicable industry standards.
One embodiment of the invention provides a cartridge heater. The cartridge includes a sheath having a first end and a second end. The first end of the sheath forms a seat. An elastomeric bushing is swaged against the seat such that it forms a mechanically bonded seal substantially preventing moisture egress into the cartridge heater. A heating element is also disposed in the sheath and is connected to leads protruding from the bushing. The heater may include crushable insulation material disposed within the sheath.